In a video coding system, video data are subject to various processing such as prediction, transform, quantization, and deblocking. Along the processing path in the video coding system, noise may be introduced and characteristics of the processed video data may be altered from the original video data due to the operations applied to video data. For example, the mean value of the processed video may be shifted. Intensity shift may cause visual impairment or artifacts, which will become noticeable especially when the intensity shift varies from picture to picture. Therefore, the pixel intensity shift has to be carefully compensated or restored to reduce the artifacts. Some intensity offset schemes have been used in the field. For example, an intensity offset scheme, termed as sample adaptive offset (SAO), classifies each pixel in the processed video data into one of multiple categories according to a context selected. The SAO scheme usually requires incorporating SAO information in the video bitstream, such as partition information to divide a picture or slice into blocks and the SAO offset values for each block so that a decoder can operate properly. Besides SAO, adaptive loop filter (ALF) is another type of in-loop filter often applied to the reconstructed video to improve video quality. Similarly, ALF information such as partition information and filter parameters has to be incorporated in the video bitstream so that a decoder can operate properly.
In a conventional coding system, picture-level in-loop filtering is often applied, where the same filter parameters are shared by all blocks in the picture. While the picture-level in-loop filtering is simple, it lacks adaptivity to local characteristics of the picture. It is desirable to develop an in-loop filtering scheme that can adapt to local characteristics of the picture. The in-loop filtering scheme with local adaptivity may require more bandwidth to convey filter information. Accordingly, it is also desirable to develop syntax that can convey filter information efficiently and/or flexibly.